Windows: If you're getting a little tired of the lock screen images in Windows 8, Chameleon is a free app that'll help you personalize it with new images or constantly-rotating images that change periodically.

Chameleon is available in the Windows App Store, and has built-in support for images from NASA's Astronomy Picture of The Day, National Geographic's photo of the day, Bing's picture of the day, and a number of other popular wallpaper sites. You can tell Chameleon to change the lock screen automatically after a given period of time in the options by itself, or you can browse the collections of images and add them to your own pool of images that Chameleon will choose from.

Windows 8 can automatically rotate wallpapers, so you don't need another app for that, but if you want a little something new and interesting every time you sit down to unlock your Windows 8 PC or tablet, Chameleon can do it for you. Plus, it's completely free.